The climatic response to the smoke was surpris¬ing. Sunlight was immediately reduced, cooling the planet to temperatures lower than any expe¬rienced for the past 1,000 years. The global aver¬age cooling, of about 1.25 degrees Celsius (2.3 degrees Fahrenheit), lasted for several years, and even after 10 years the temperature was still 0.5 degree C colder than normal. The models also showed a 10 percent reduction in precipitation worldwide. Precipitation, river flow and soil moisture all decreased because blocking sun¬ light reduces evaporation and weakens the hydrologic cycle. Drought was largely concen¬trated in the lower latitudes, however, because global cooling would retard the Hadley air cir-culation pattern in the tropics, which produces a large fraction of global precipitation. In criti¬cal areas such as the Asian monsoon regions, rainfall dropped by as much as 40 percent.
The cooling might not seem like much, but even a small dip can cause severe consequences. Cooling and diminished sunlight would, for ex¬ample, shorten growing seasons in the midlati¬tudes. More insight into the effects of cooling came from analyses of the aftermaths of massive volcanic eruptions. Every once in a while such eruptions produce temporary cooling for a year or two. The largest of the past 500 years, the 1815 Tambora eruption in Indonesia, blotted the sun and produced global cooling of about 0.5 de¬gree C for a year; 1816 became known as “The Year without a Summer” or “Eighteen Hundred and Froze to Death.” In New England, although the average summer temperature was lowered only a few degrees, crop-killing frosts occurred in every month. After the first frost, farmers re¬planted crops, only to see them killed by the next frost. The price of grain skyrocketed, the price of livestock plummeted as farmers sold the animals they could not feed, and a mass migration began from New England to the Midwest, as people fol¬lowed reports of fertile land there. In Europe the weather was so cold and gloomy that the stock market collapsed, widespread famines occurred and 18-year-old Mary Shelley was inspired to write Frankenstein.
Certain strains of crops, such as winter wheat, can withstand lower temperatures, but a lack of sunlight inhibits their ability to grow. In our scenario, daylight would filter through the high smoky haze, but on the ground every day would seem to be fully overcast. Agronomists and farmers could not develop the necessary seeds or adjust agricultural practices for the rad¬ically different conditions unless they knew ahead of time what to expect.
In addition to the cooling, drying and dark¬ness, extensive ozone depletion would result as the smoke heated the stratosphere; reactions that create and destroy ozone are temperature-depen¬dent. Michael J. Mills of the University of Colo¬rado at Boulder ran a completely separate climate model from Robock’s but found similar results for smoke lofting and stratospheric temperature changes. He concluded that although surface temperatures would cool by a small amount, the stratosphere would be heated by more than 50 degrees C, because the black smoke particles ab¬sorb sunlight. This heating, in turn, would mod¬ify winds in the stratosphere, which would carry ozone-destroying nitrogen oxides into its upper reaches. Together the high temperatures and ni¬trogen oxides would reduce ozone to the same dangerous levels we now experience below the ozone hole above Antarctica every spring. Ultra¬violet radiation on the ground would increase significantly because of the diminished ozone.
Less sunlight and precipitation, cold spells, shorter growing seasons and more ultraviolet ra¬diation would all reduce or eliminate agricultur¬al production. Notably, cooling and ozone loss would be most profound in middle and high lat¬itudes in both hemispheres, whereas precipita¬tion declines would be greatest in the tropics.
The specific damage inflicted by each of these environmental changes would depend on partic¬ular crops, soils, agricultural practices and re¬gional weather patterns, and no researchers have completed detailed analyses of such agricultural responses. Even in normal times, however, feed¬ing the growing human population depends on transferring food across the globe to make up for regional farming deficiencies caused by drought and seasonal weather changes. The total amount of grain stored on the planet today would feed the earth’s population for only about two months [see “Could Food Shortages Bring Down Civiliza¬tion?” by Lester R. Brown; Scientific Ameri¬can, May]. Most cities and countries have stock¬piled food supplies for just a very short period, and food shortages (as well as rising prices) have increased in recent years. A nuclear war could trigger declines in yield nearly everywhere at once, and a worldwide panic could bring the glob¬al agricultural trading system to a halt, with se¬vere shortages in many places. Around one billion people worldwide who now live on marginal food supplies would be directly threatened with star¬vation by a nuclear war between India and Paki¬stan or between other regional nuclear powers.
Source of Information : Scientific American January 2010